Studies proposed in this application are designed to develop an understanding of the molecular and cellular basis for age-related changes in lipid metabolism that impact the development of Alzheimer's disease (AD). To achieve this goal we will use a cross-disciplinary approach that will combine unique sets of clinical data and human biospecimens together with mechanistic hypotheses testing using a mouse model of AD with a relevant targeted genetic abnormality in phospholipid metabolism. There is strong evidence that phospholipid metabo- lism is abnormal in AD brain and there are data indicating that prodromal and frank clinical AD have correlates in the blood plasma lipidome that include apparent abnormalities of the turnover of phosphatidylcholine (PC) ? a major constituent of all biological membranes, including those in neurons and glial cells. This is reflected by reduced plasma levels of certain molecular species of PC containing eicosapentaenoic- [EPA, 20:5n-3] and docosahexaenoic acid [DHA, 22:6n-3] in AD. Together, the data suggest the hypothesis that AD pathophysiol- ogy may be characterized by lipidomic abnormalities that occur early in the disease and encompass both the periphery (as reflected by plasma lipidomic changes) and the brain. However, heretofore it has not been possi- ble to address this idea directly due to the lack of an appropriately optimized scientific and clinical infrastructure that incorporates blood and brain samples. We propose to test this hypothesis by interdisciplinary approaches that will employ sets of data and biorepositories of plasma and postmortem brain tissue derived from the par- ticipants of the Framingham Heart Study (FHS) population. The FHS has epidemiology of dementia and brain donation programs (over 200 brains available), resulting in longitudinal clinical and neuropsychological data for subjects whose diagnoses range from cognitively intact to mild cognitive impairment to dementia. The neuro- pathological diagnoses of these subjects range from normal to advanced AD. The FHS has food frequency questionnaire data and has collected plasma samples longitudinally over time from these subjects, making it an optimal resource to identify abnormalities in the metabolism of lipids, associated with cognitive impairment and AD pathology, that may serve as biomarkers and targets for therapy. The polyunsaturated species of PC (such as PC-DHA) are primarily synthesized by the enzyme phosphatidylethanolamine N-methyltransferase (PEMT). PEMT abnormalities reduce plasma PC-DHA concentrations in humans and have been associated with increased risk of AD. Therefore, abnormal PEMT activity may contribute to AD pathophysiology. We pro- pose the following aims: 1) To identify lipidomic- and PEMT activity changes associated with age and AD de- velopment in brain tissue; 2) To determine lipidomic profiles of serial, stored plasma samples obtained in life from the above subjects, and correlate the plasma lipidomic data with the information obtained in Aim 1 and 3) To determine the effect of PEMT activity and PC-DHA levels on the development of AD-like pathology in AD model mice with 2, 1, or 0 copies of the Pemt gene, for mechanistic hypothesis testing.